This invention is in the field of integrated circuits, and more specifically is directed to the packaging of integrated circuits.
Portability has become commonplace in the field of electronic systems. For example, laptop or notebook-size computers are now commonly used, as are portable communications systems such as cellular telephones and cellular facsimile machines. As is evident to those in the art, portability of an electronic system is maximized when the circuit boards are as small and as densely populated with circuits as possible. Portability also generally requires the installation of batteries on the circuit boards, for use as primary or backup power sources.
The use of battery backup power sources has also become common in non-portable electronic systems. For example, many personal computers and workstations include battery-powered memory for purposes of maintaining an integral clock or calendar function. Such non-portable electronic systems also benefit from small form factor and densely populated circuit boards.
Modern electronic assembly technology utilizes surface mountable integrated circuit packages for high density populating of circuit boards. The leads of surface mountable packages are of such shape and size that they may be soldered to the surface of a circuit board by way of a solder reflow process, in contrast to the leads of dual-in-line packages ("DIP") which extend through holes in the circuit board for solder-connection on the reverse side. Surface mounting increases the population density of the circuit board because only traces on the surface of the board (rather than throughout the board) need be dedicated for connection to the integrated circuit. In addition, the size of the package may be reduced greatly from that required for a DIP package for the same circuit, especially where the number of leads extending from the circuit is large. Surface mount technology thus allows the form factor of the circuit board to be greatly reduced from that achievable for the same electronic function utilizing DIP circuit packages.
Conventional surface mountable integrated circuit packages exist in various forms. Ceramic hermetically sealed surface mountable packages generally are of the leadless chip-carrier type, or alternatively have solder bump or solder ball leads. Plastic encapsulated or molded surface mountable packages may be of the leaded chip carrier type, with J-leads or with flat leads extending from the body of the package.
Conventional surface mounting technology attaches the integrated circuit packages to the circuit board by way of a solder reflow process where the solder flows adjacent to the circuit package itself, rather than on the other side of the circuit board from the circuit package. As such, the temperature to which the surface mountable circuit package is exposed in solder reflow is significantly greater than that to which a DIP package is exposed in its mounting to a circuit board. While brief exposure to such a high temperature (from 180.degree. C. to as high as 230.degree. C.) is not believed to significantly degrade circuit reliability, such high temperature exposure has been observed to seriously degrade batteries, such as conventional lithium or silver-zinc type cells.
Because of this temperature degradation of conventional batteries, it has been heretofore impractical, if not impossible, to reliably integrate a battery into a surface mountable integrated circuit package. As such, according to conventional packaging technology, batteries must be inserted into battery-powered circuits in surface mount applications after solder reflow mounting of the integrated circuit package to the circuit board, complicating and increasing the cost of manufacture of the electronic system.
By way of further background, DIP packages have been produced which include a battery integrated therewithin, due to the reduced temperatures to which the DIP circuit package is exposed during soldering (the intervening circuit board being a thermal insulator).
It is therefore an object of the present invention to provide a surface mountable integrated circuit package within which a battery may be integrated and which can withstand the temperature excursion of the solder reflow process.
It is a further object of the present invention to provide such a package of the molded encapsulated type.
It is a further object of the present invention to provide such a package of the hermetically sealed type.
It is a further object of the present invention to provide a method of fabricating such a package.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.